ELI5: Why is a hard drive in a computer entitled "C:" by default? Shouldn't it be called "Hard Drive A" or "Drive A"?

Back in the day on IBM Compatible PCs (the banner by which all modern non-apple* machines come under), floppy drives were the main storage medium, most machines didn't even have hard drives.

That's why, even now, if you install a floppy disk drive onto a PC, it'll get the Drive A designation.

There's a PC where I work that's old enough to have both the larger 5.25 inch floppy disk drive, and the somewhat newer 3.5 inch floppy disk drive, as well as a (20MB!) hard drive.

The 5.25 floppy is Drive A, the 3.5 inch floppy is Drive B, and the hard drive is Drive C.

This convention survives until now mainly in the name of hardware compatibility.

I realize that nowadays the term IBM compatible is a bit of a misnomer. I refer to the period of computing after IBM created the PC-AT and tons of other companies began copying it, thus establishing the standard of the "IBM Compatible". The modern non apple computer still sticks largely to the roots of that old standard, which includes drive lettering - the point of the original question. Technically speaking nowadays yes you can run the vast majority of PC software on a Mac with appropriate virtual environments and so on now they use processors of a similar architecture, but during the period of computing I refer to, this most certainly wasn't the case.

Well okay that's a bit of an old fashioned view on things since Apple began using Intel based hardware. I'm talking in the PowerPC days, which is really when "IBM Compatible" was still a relevant term.

Nowadays, they are, although this has not always been the case historically. The key difference is that they use a different operating system. In DOS, each drive forms the root of its own filesystem. In UNIX, there is a single filesystem with a single root, and all drives are subtrees of that filesystem.

It's easier to keep using C: because the last version of the OS used C than it it to risk confusing users & breaking old programs. Even if the old programs are poorly written, customers will blame the OS because that's what they saw changed.

As people have mentioned, it's backwards compatibility. However, it's backwards compatibility with Windows and DOS. Unix and Linux do not use drive letters to indicate a drive, they use /dev/storagename for hard drives and /media for removable storage like floppy disks and USB sticks.